The present invention pertains to a search loop assembly for a hand-held metal detector. More specifically, it pertains to a housing portion of the search loop assembly, containing the search loop coils, and to prevention of the development of significant electrostatic charges on the housing.
A transmit coil in a search loop assembly of a hand-held metal detector is driven electrically to generate a periodically changing magnetic field. A receive coil, also in the search loop assembly, detects changes in the magnetic field so generated; these changes may indicate the presence of desirable metallic objects within the magnetic field.
To avoid extraneous disturbance of the magnetic fields surrounding the receive and transmit coils of such loop assemblies, housings for them have previously been constructed of nonconductive plastics. A plastic search loop housing, however, can accumulate an electrostatic charge, as when brushed over grass or sand during use of a metal detector. The irregular discharge of such charges can mask changes in the magnetic field resulting from desirable target materials, making it difficult to use the metal detector reliably.
The problem of static electric charge buildup in such plastic housings has been solved by providing a path for conduction of electricity, which is associated with the housing and is connected to a ground or common potential so as to conduct electrical charges away from the search loop housing. A Faraday shield of metallic conductors would cause an undesired effect on the performance of a metal detector. In the past, therefore, static charge drainage for metal detector search loop assemblies has been provided by spraying the interior of a plastic housing with a layer of a non-metallic conductive material, such as C3010 external black conductive coating available from Pierce & Steven Chemical Corp. of Buffalo, N.Y. Such a coating forms an electrically conductive layer having some electrical resistance, which is desirable, since a certain amount of resistance apparently prevents generation of eddy currents in the coating which can adversely affect the way the receive coil "sees" the area being searched. Such a sprayed-on layer of electrically conductive material, however, may later crack and peel, diminishing its beneficial effects as a drain for static charges. Also, spraying such material onto the housing adds costly labor to the manufacture of search loop assemblies.
The use of various electrically conductive materials to form shields for electronic equipment is known. For example, use of metal sheets or foils, or plastic material filled with metallic conductors such as metal-coated glass fibers is known for electrostatic or electromagnetic shielding in some applications. However, such materials, since they contain metal, would distort magnetic fields of the types used by metal detectors, leaving those materials useless for the present purpose. So long as metallic particles do not touch one another within the housing material, no static discharge drainage is provided, but the material of such a housing will affect the magnetic field, appearing to the metal detector as a large metallic target.
Carbon is another conductive material that is known as a filler, in the form of fibers, for making plastic materials electrically conductive for sue as shields, as disclosed in Soens U.S. Pat. No. 4,664,971. Carbon in the amorphous state, in the form of relatively short fibers sometimes called electrical grade carbon, can be added to plastic. Such carbon fiber-filled plastics are commercially available and are recommended for shielding against electromagnetic interference. Unfortunately, when plastic containing such carbon fibers in low concentrations is molded the carbon fiber material tends to agglomerate into isolated groups, lacking uniform distribution and predictable conductivity. The groups of carbon particles, furthermore, are detrimental to metal detector performance. Increasing the concentration of such electrical grade carbon as a filler in a plastic, to a high enough concentration that the carbon groups interconnect reliably, results in a plastic material having such a high conductivity that eddy currents can be generated in the housing. Such plastic material filled with carbon also does not perform structurally as well as desired for construction of a search loop housing not susceptible to static electric charge accumulations.
In a metal detector search loop assembly the transmit coil, receive coil, and a feedback coil must be maintained in a fixed spatial arrangement with one another and with respect to a static shield. A low resistance static shield causes an offset of the effect of the driven coils' field on the receive coil of a metal detector, by comparison with the absence of a static charge-dissipating or Faraday shield. Changes occurring in this spatial arrangement during metal detector use can cause deceptive signals to be generated in the receive coil.
Because of the usual attachment of a handle to a search loop assembly, the mechanical forces applied to the housing during use of a metal detector could easily cause distortion of a light, flexible, plastic housing and relative movement of the coils carried in the housing and of a conductive coating used to conduct electricity to eliminate static charge accumulations. Any such movement can cause aberrations of the signals produced in the receive coil of the metal detector and thus could lead to misinterpreted signals. Previously known search loop assembly construction techniques have solved this problem with varying degrees of success. One solution is to make a housing of thicker plastic, thus making the housing more rigid. Unfortunately, a housing of thicker material also is heavier, which is undesirable in a metal detector for hand-held use. Another solution is to support the coils on a separate circular base of foam plastic which contacts the plastic housing only at the base's perimeter. In this type of structure a flexing housing is less likely to change the spatial relationships among the coils. Use of such a separate base for supporting the coils, however, requires separate steps for winding the coils, arranging the coils on the base, and assembling the base and the housing as a unit, adding to the cost of manufacturing a search loop assembly of that type.
What is needed then is a simply constructed housing for the search loop assembly of a metal detector, including a static charge dissipating shield which is not likely to be mechanically eroded during use, preferably having neutral buoyancy for underwater use, and, without being too heavy, having sufficient rigidity to resist mechanical deformation which might affect search loop performance.